An optical OFDM technique which applies an OFDM technique to optical communication is evaluated as a technique that has large tolerance in elements deteriorating an optical signal quality, such as chromatic dispersion and polarization mode dispersion of an optical fiber and can easily compensate for the chromatic dispersion and polarization mode dispersion in a receiver. Therefore, various researches into the optical OFDM technique have been performed.
An orthogonal frequency division multiplexing (OFDM) technique is a communication technique that allocates a plurality of subcarriers perpendicular to each other and transmits data through each of the subcarriers at a relatively low symbol rate in order to transmit a signal at a high transmission speed. The OFDM communication technique as a technique that can cope with multiple fading effects with high spectrum efficiency has been generally used in WiMAX, Wireless LAN, ADSL, a digital radio and video broadcasting system.
Meanwhile, in a coherent optical OFDM, a laser beam of a local oscillator having relatively large power and a received signal interfere with each other to be down-converted and are converted into an electrical signal by an optical detector. Phase noise of the laser used during the above process may have a large influence on system performance. Since the symbol rate in the OFDM system is much lower than that of a single carrier system, the OFDM signal may be influenced much more by the phase noise and phase noise compensation requirements may also be more strict. In order to compensate for the influence by the phase noise of the laser, several methods including a method of using OFDM pilot subcarriers and a method using an RF pilot tone are presented.
One method adopts a method of using pilot subcarriers in an OFDM symbol in order to compensate for phase noise of a transmitter and a receiver in the coherent optical OFDM (“Phase Estimation for Coherent Optical OFDM”, IEEE Photonics Technology Letters, Vol. 19, No. 12, pp. 919-921, 2007). In this method, several pilot subcarriers are allocated in addition to the OFDM data subcarriers in an OFDM symbol spectrum and phases of the pilot subcarriers are estimated in the receiver. A phase estimating method using the pilot subcarrier uses a plurality of parallel subcarriers in the OFDM system and is difficult to be implemented in the general single carrier system. Phase variation estimated in one OFDM symbol represents an average value of differences in the received phase and the transmitted phase in the pilot subcarriers. Therefore, by multiplying received OFDM data by a complex conjugate of an estimated phase variation, phase noise of the received OFDM data is compensated. In this method, since it is assumed that the phase variation is uniform in one OFDM symbol, performance deteriorates when noise is generated by rapid phase variation.
Another method adopts a method of compensating phase noise by adding an RF-pilot tone to a middle part (DC) of the OFDM band in the transmitter (“Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF,” IEEE Journal of Lightwave Technology Letter, vol. 26, no. 1, pp. 6-15, 2008). Since the RF-pilot tone is distorted in the completely same manner as an OFDM signal by the phase noise, distortion of the OFDM signal can be compensated. In the method of compensating the phase noise by adding the RF pilot tone, since the RF pilot tone is positioned at the center (DC) of the OFDM signal, additional bandwidth or hardware is not required. Further, since phase noise is compensated in each received sample, the non-uniform phase variation during one OFDM symbol can be compensated.
The present disclosure provides a method of adding the additional pilot tone to not the DC but a frequency range without the OFDM data subcarriers in the OFDM band and compensating for the phase noise per sample unlike the method using the pilot subcarrier among the OFDM data subcarriers.